1. Field of the Invention
The present invention relates to a pitch control apparatus, and, more particularly, to a pitch control apparatus which changes the frequency of an audio signal to a desired frequency so as to effect a pitch control between an original musical tone and a reproduced musical tone.
2. Description of Background Information
There are some conventional pitch control apparatuses configured to write data, which is obtained by sampling an analog input signal, sequentially into a ring buffer memory, read data in a different cycle from a writing cycle, and sequentially demodulate the read data, thus changing the pitch of the signal. Such a pitch control apparatus controls the amount of read-out data in the following manner. If a cycle for reading data from the ring buffer memory is longer than the writing cycle (i.e., in the case of lowering the pitch), data stored in the buffer memory is read, partially overlapping the previously read data, by every predetermined period. If the data-reading cycle is shorter than the writing cycle (i.e., in the case of raising the pitch), on the other hand, some data is skipped and the succeeding data is read out. If both the reading cycle and the writing cycle are variable, skipping data or reading data twice is not necessary. If the contents of the data, when being skipped or read out twice, have less relativity to those of the previous or succeeding data, discontinuous points would occur in a musical tone to be reproduced. A so-called cross-fading technique is used to reduce these deficiencies. In the case that the reading cycle is shorter than the writing cycle, the difference between a write point W and a read point R of the ring buffer memory, d.sub.R-W, is usually greater than a predetermined value d.sub.th, as shown in FIG. 1A. It is assumed that both points advance clockwise. When d.sub.R-W &lt;d.sub.th, data is read out at a point R', away clockwise from the read point R by the predetermined value d.sub.th, as shown in FIG. 1B, a fade-out process is linearly performed on the read data at the read point R, and a fade-in process is performed on the data at the read point R'. These pieces of data are then added together, realizing the cross-fading.
Since the number of read points changes from one to two in the cross-fade period, however, depending on the frequency components of a signal, their phases may become opposite to each other to be canceled out, or the frequency components, when in phase, raise their levels. This causes a large time-dependent change in the frequency response as shown in FIG. 2, causing a problem of generating a so-called tremolo tone.
The larger the size or the memory capacity of the ring buffer memory, the greater the predetermined value d.sub.th can be set. With a larger memory size, the number of dips for the frequency response in FIG. 2 increases, while the interval between the dips and the width of the dip itself become narrower. Since the probability of the frequency of an audio signal being equal to that of the dips becomes lower, therefore, the occurrence of tremolo tones is considered to be suppressed.
Even if intervals between a plurality of read points can be set wide, however, the time duration between pieces of data to be read out becomes larger. This causes a reverberation phenomenon. For example, in the case of the sound of a percussion or the like, one beat in the original tone appears to sound two or more beats after the pitch control. While this reverberation phenomenon occurs only during the cross-fade in the conventional pitch control apparatus, in a pitch control apparatus which is configured to always designate a plurality of read points in the ring buffer memory to read data therefrom, and multiply each piece of the read data by a coefficient and then add them together to acquire output data, however, the reverberation phenomenon would always occur if the ring buffer memory has a large memory capacity.